As shown in FIG. 16A in plan view and as shown in FIG. 16B in sectional view taken along the line D-D of FIG. 16A, an optical waveguide W19 in general includes: an under cladding 1; linear cores 2 for light propagation protruding in a predetermined pattern and formed on a surface of the under cladding 1; and an over cladding 3 formed on the surface of the under cladding 1 so as to cover the cores 2. The optical waveguide W19 is configured such that light enters a light entrance portion 2a disposed in a first end portion of each of the cores 2 and that the light exits alight exit portion 2b disposed in a second end portion of each of the cores 2. Specifically, a light exit member 10 such as an optical fiber, a light-emitting element, or the like is connected to the light entrance portion 2a disposed in the first end portion of each core 2, and light exiting the light exit member 10 enters the light entrance portion 2a disposed in the first end portion of each core 2 and is propagated in each core 2 to the light exit portion 2b disposed in the second end portion of each core 2 while being reflected repeatedly from an interface with the under cladding 1 and an interface with the over cladding 3 although not shown. In FIG. 16A, optical fibers are shown as the light exit members 10. In FIG. 16B, the reference numeral 7 designates a substrate for use in the production of the optical waveguide W19.
In actuality, there are cases in which the light exit member 10 and the light entrance portion 2a of each core 2 are slightly different in size from each other or slightly misaligned with each other, with the light exit member 10 and the light entrance portion 2a of each core 2 connected to each other. In the case where the light exit member 10 and the light entrance portion 2a of one of the cores 2 are slightly different in size from each other (with reference to a left-hand end portion in FIG. 16A), part of the light exiting the light exit member 10 sometimes does not enter the one core 2 but enters the over cladding 3 surrounding the one core 2 (with reference to dash-double-dot arrows L1). In the case where the light exit member 10 and one of the cores 2 are slightly misaligned with each other (with reference to a right-hand end portion in FIG. 16A), part of the light exiting the light exit member 10 also sometimes does not enter the one core 2 but enters the over cladding 3 surrounding the one core 2 (with reference to dash-double-dot arrows L2).
In the optical waveguide W19 including the cores 2 for light propagation arranged in side-by-side relation, the entrance of part of the light exiting the light exit member 10 into the over cladding 3, rather than a predetermined one of the cores 2, in the aforementioned manner causes what is called “crosstalk” that is a situation in which the light enters a core 2 adjacent to the predetermined core 2. The light entering the adjacent core 2 is noise (N) for light (signal S) propagating in the adjacent core 2, and decreases the S/N ratio to make optical communications unstable.
As shown in FIG. 17 in plan view, there are cases in which a light entrance member 20 such as an optical fiber, a light-receiving element, or the like which receives light exiting the light exit portion 2b disposed in the second end portion of each core 2 is connected to the light exit portion 2b. In such cases, crosstalk also sometimes occurs. Specifically, there are cases in which light leaks from a predetermined one of the cores 2 into the over cladding 3 and the leaking light enters the light entrance member 20 connected to the light exit portion 2b disposed in the second end portion of a core 2 adjacent to the predetermined core 2 (with reference to dash-double-dot arrows L3 and L4). In a left-hand end portion of FIG. 17, the light entrance member 20 and the light exit portion 2b of a core 2 are slightly different in size from each other. In a right-hand end portion of FIG. 17, the light entrance member 20 and a core 2 are slightly misaligned with each other.
To solve such a problem, an optical waveguide W20 has been proposed as shown in FIG. 18A in plan view and as shown in FIG. 18B in sectional view taken along the line E-E of FIG. 18A (see PTL 1, for example). In the optical waveguide W20, dummy cores 2D made of the same material as the cores 2 and not used for light propagation are provided between adjacent ones of the cores 2 for light propagation to thereby suppress crosstalk. Like the cores 2, the dummy cores 2D in this optical waveguide W20 have a refractive index higher than the refractive indices of the under cladding 1 and the over cladding 3. In the optical waveguide W20, it is hence intended that light entering the dummy cores 2D after entering the over cladding 3, rather than the cores 2, is less prone to leak from the dummy cores 2D, although not shown. For the purpose of clarifying the arrangement of the cores 2 and the dummy cores 2D, the cores 2 and the dummy cores 2D are shaded by means of broken diagonal lines in FIG. 18A, and the diagonal lines for the dummy cores 2D are spaced more widely than those for the cores 2.